Camshaft and follower geometry

ABSTRACT

An internal combustion engine includes a camshaft with a lobe rotatable about a camshaft axis and a follower and which follows the lobe and is reciprocated along a follower axis by the lobe. A contact force is generated between the lobe and the follower such that the contact force is cyclic between a minimum contact force and a maximum contact force. The follower axis is offset from the camshaft axis such that the lobe contacts the follower at a contact point that is substantially aligned with the follower axis when the maximum contact force is generated between the lobe and the follower. In this way, the side load on the follower is minimized.

TECHNICAL FIELD OF INVENTION

The present invention relates to an internal combustion engine with a camshaft having a lobe and a follower which follows the lobe; more particularly to such an internal combustion engine in which the follower imparts a pumping stroke on a pumping plunger of a fuel pump; and more particularly to such an internal combustion engine which minimizes tipping forces imparted on the follower.

BACKGROUND OF INVENTION

It is known in art of internal combustion engines to provide a fuel pump which is driven by a camshaft which rotates about a camshaft axis. U.S. Pat. No. 7,568,469 to Tokuo et al. shows an example of such an arrangement. The camshaft includes a lobe which is followed by a follower of the fuel pump and which reciprocates about a follower axis which is typically perpendicular to and aligned with the camshaft axis. The lobe reciprocates the follower through one or more pumping strokes for each rotation of the camshaft, thereby reciprocating a pumping plunger of the fuel pump to pressurize the fuel. Due to geometry of the lobe, the point of contact between the lobe and the follower varies in a cyclic patter from being aligned with the follower axis to being eccentric to the follower axis. A contact force between the follower and the lobe is generated which also varies in a cyclic pattern as a function of the rotational position of the lobe such that the highest contact force occurs when the contact point is not aligned with the follower axis which results in a side load being applied to the follower. This side load may result in reduced durability and may require followers which are manufactured with increased strength to be capable of accommodating the side load, thereby increasing manufacturing costs.

What is needed is an internal combustion engine which minimizes or eliminates one or more of the shortcomings as set forth above.

SUMMARY OF THE INVENTION

Briefly described, an internal combustion engine includes a camshaft with a lobe rotatable about a camshaft axis and a follower which follows the lobe and which is reciprocated along a follower axis by the lobe. A contact force is generated between the lobe and the follower such that the contact force is cyclic between a minimum contact force and a maximum contact force. The follower axis is offset from the camshaft axis such that the lobe contacts the follower at a contact point that is substantially aligned with the follower axis when the maximum contact force is generated between the lobe and the follower. In this way, the side load on the follower is minimized.

Further features and advantages of the invention will appear more clearly on a reading of the following detailed description of the preferred embodiment of the invention, which is given by way of non-limiting example only and with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

This invention will be further described with reference to the accompanying drawings in which:

FIG. 1 is a schematic drawing of an internal combustion engine in accordance with the invention;

FIG. 2 is an enlarged portion of FIG. 1 showing a lobe of a camshaft and a follower which follows the lobe;

FIG. 3 is a graph showing lift of the follower vs. angle of rotation of the lobe, contact force between the follower and the lobe vs. angle of rotation of the lobe, and contact point eccentricity relative to follower axis vs. angle of rotation of the lobe; and

FIG. 4 is the enlarged portion of FIG. 2 now showing an alternative follower.

DETAILED DESCRIPTION OF INVENTION

In accordance with a preferred embodiment of this invention and referring to FIGS. 1 and 2, an internal combustion engine 10 is shown. Internal combustion engine 10 includes a fuel pump 12 which is driven by a camshaft 14 of internal combustion engine 10. Fuel pump 12 is used to deliver pressurized fuel to one or more fuel injectors 16, which may, for example only, inject the pressurized fuel directly into combustion chambers (not shown) of internal combustion engine 10.

Fuel pump 12 generally includes a fuel inlet 18, an inlet valve 20, a pressure chamber 22, a pumping plunger 24, a return spring 25, an outlet valve 26, and a fuel outlet 28 which may be in fluid communication with a fuel rail 30 to which fuel injectors 16 are connected and receive pressurized fuel from. Fuel inlet 18 receives fuel from a fuel source, for example a fuel tank (not shown) with a lift pump (not shown) at a relatively low pressure. Inlet valve 20 may be, for example, a solenoid operated valve which is controlled by a controller 32. Controller 32 may receive input from a pressure sensor 34 which supplies a signal indicative of the pressure of the fuel being supplied to fuel injectors 16. While pressure sensor 34 is shown arranged to read the fuel pressure within fuel rail 30, it should be understood that pressure sensor 34 may be positioned at other locations that are indicative of the pressure of the fuel being supplied to fuel injectors 16. Controller 32 sends signals to inlet valve 20 to open and close inlet valve 20 as necessary to achieve a desired fuel pressure at pressure sensor 34 as may be determined by current and anticipated engine operating demands. When inlet valve 20 is opened, fuel from fuel inlet 18 is allowed to flow into pressure chamber 22.

Pumping plunger 24 is slidably received within a plunger bore 36 that is in fluid communication with pressure chamber 22. Pumping plunger 24 is reciprocated within plunger bore 36 along a plunger axis 38 by camshaft 14 and return spring 25 as will be described later. Pumping plunger 24 includes a follower 40 at an axial end thereof that is distal from pressure chamber 22 for following camshaft 14 as will also be described later. Follower 40 reciprocates within a follower bore 41 which may be, for example only, a bore in the engine block, intake manifold, or cylinder head of internal combustion engine 10 or may alternatively be a bore in a separate housing of fuel pump 12. Follower 40 is guided by follower bore 41 which is substantially coaxial with plunger bore 36. After inlet valve 20 has been closed, reciprocation of pumping plunger 24 toward pressure chamber 22 causes the fuel within pressure chamber 22 to be compressed. The pressurized fuel causes outlet valve 26 to open, thereby allowing pressurized fuel to be expelled from fuel outlet 28 to fuel rail 30 and fuel injectors 16.

Camshaft 14 rotates about a camshaft axis 42 and includes a lobe 44 which is followed by follower 40 of pumping plunger 24. As shown in FIGS. 1 and 2, follower 40 forms a sliding interface with lobe 44; however, as shown in FIG. 4, follower 40 may include a roller 43 which forms a rolling interface with lobe 44. Camshaft axis 42 is arranged to be substantially at a 90° angle relative to plunger axis 38; however, plunger axis 38 is offset relative to camshaft axis 42 such that plunger axis 38 does not intersect camshaft axis 42 as will be described later in greater detail. One rotation of lobe 44 causes pumping plunger 24 to complete three pumping cycles, i.e. pumping plunger 24 is reciprocated from a minimum displacement to a maximum displacement (pumping stroke) and back again to minimum displacement (refill stroke) in one pumping cycle. Lobe 44 includes three compression flanks 46 a, 46 b, 46 c which each move pumping plunger 24 from the minimum displacement toward the maximum displacement, three refill flanks 48 a, 48 b, 48 c which each move pumping plunger 24 from the maximum displacement toward the minimum displacement, and three noses 50 a, 50 b, 50 c between respective compression flanks 46 a, 46 b, 46 c and refill flanks 48 a, 48 b, 48 c which each provide maximum displacement to pumping plunger 24. Base portions 52 a, 52 b, 52 c separate adjacent respective compression flanks 46 a, 46 b, 46 c and refill flanks 48 a, 48 b, 48 c. Base portions 52 a, 52 b, 52 c provide minimum displacement to pumping plunger 24 and may define the point at which the direction of movement of pumping plunger 24 reverses. While camshaft 14 has been illustrated as having three compression flanks 46 a, 46 b, 46 c, three refill flanks 48 a, 48 b, 48 c, and three noses 50 a, 50 b, 50 c, it should be understood that a lesser or greater number may be provided, thereby decreasing or increasing the number of pumping cycles that are completed during one rotation of camshaft 14.

A description of the rotation of camshaft 14 will now be given. For this description, rotation positions will be assigned as follows: 0° is the center of base portion 52 a, 63° is nose 50 a, 120° is the center of base portion 52 b, 183° is nose 50 b, 240° is the center of base portion 52 c, and 303° is nose 50 c; however, it should be understood that these values are exemplary only. A contact force is generated between lobe 44 and follower 40 as a result of return spring 25 urging follower 40 into contact with lobe 44 and also as a result of the pressure being elevated within pressure chamber 22 during the pumping stroke. This contact force is cyclic between a minimum contact force and a maximum contact force and varies as a function of the rotational position of camshaft 14. As shown by contact force trace 54 in FIG. 3, the maximum contact force occurs at about 22°, 142°, and 262° with elevated contact forces surrounding the points of maximum contact force between about 15° and about 45°, between about 135° and about 165°, and between about 255° and about 285° respectively. Consequently, the maximum contact force and the elevated contact forces occur during about a range of 30° on each compression flank 46 a, 46 b, 46 c. Also as can be seen in FIG. 3, the maximum contact forces do not coincide with the maximum lifts of follower 40 which occur when noses 50 a, 50 b, 50 c contact follower 40 as illustrated by lift trace 56. The contact point between follower 40 and lobe 44 is also cyclic and varies as a function of the rotational position of camshaft 14 as illustrated by contact point eccentricity trace 58. When camshaft 14 is rotated to 0°, 120°, or 240°, lobe 44 contacts follower 40 at a point of contact that is offset from plunger axis 38 a magnitude that is substantially equal to the magnitude of offset of plunger axis 38 relative to camshaft axis 42. Since the point of contact between follower 40 and lobe 44 is offset from plunger axis 38, a tipping force is generated which tries to tip follower 40, thereby applying a side load to follower 40 within follower bore 41; however, since the contact force is relatively low, the tipping force and side load is also relatively low. As camshaft 14 is rotated counterclockwise, as viewed in the drawings, from 0°, 120°, or 240°, the point of contact between follower 40 and lobe 44 moves toward plunger axis 38, i.e. the eccentricity becomes smaller. When camshaft 14 is rotated to generate the maximum contact force between follower 40 and lobe 44, the point of contact between follower 40 and lobe 44 is substantially aligned with plunger axis 38. Since the point of contact between follower 40 and lobe 44 is substantially aligned with plunger axis 38, there is substantially no tipping force generated which tries to tip follower 40, thereby minimizing the side load placed on follower 40 within follower bore 41. As used herein, substantially aligned is understood to be within about ±5% of the total eccentricity of plunger axis 38 and the point of contact between follower 40 and lobe 44.

While an exemplary lobe 44 has been described herein, it should be understood that other lobe geometries are contemplated. For example only, the location, duration, lift properties, and quantities of compression flanks 46 a, 46 b, 46 c; refill flanks 48 a, 48 b, 48 c; noses 50 a, 50 b, 50 c; and base portions 52 a, 52 b, 52 c may be altered in order to obtain desired operating performance of fuel pump 12.

While this invention has been described in terms of preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow. 

We claim:
 1. An internal combustion engine comprising: a camshaft having a lobe rotatable about a camshaft axis; and a follower which follows said lobe and is reciprocated along a follower axis by said lobe whereby a contact force is generated between said lobe and said follower such that said contact force is cyclic between a minimum contact force and a maximum contact force; wherein said follower axis is offset from said camshaft axis such that said lobe contacts said follower at a contact point that is substantially aligned with said follower axis when said maximum contact force is generated between said lobe and said follower, thereby minimizing side load on said follower.
 2. An internal combustion engine as in claim 1 wherein said follower reciprocates within and is guided by a follower bore.
 3. An internal combustion engine as in claim 1 wherein reciprocating motion of said follower is translated to a pumping plunger which reciprocates within a plunger bore.
 4. An internal combustion engine as in claim 3 wherein said plunger bore is in fluid communication with a pumping chamber and reciprocation of said pumping plunger pressurizes fuel within said pumping chamber.
 5. An internal combustion engine as in claim 4 wherein said follower reciprocates within and is guided by a follower bore.
 6. An internal combustion engine as in claim 5 wherein said follower bore is coaxial with said plunger bore. 